It is known to place in front of roof overhangs or canopies of limited height and beneath which vehicles travel, as in fast food restaurants, pivotal arms designed to be struck by an over-height vehicle with the expectation that the sound of the impact of the vehicle with the arm will serve as a warning to the driver that he is about to strike the overhang or canopy. Should the sound of the impact not be heard or heeded, particularly where the over-height is caused by a relatively soft roof-top load, say, duffle bags, the sound might be muffled while such a load may still be massive enough to damage a canopy extensively, possibly bringing it down and severely damaging the vehicle.
This problem has been recognized in the patent to Hicks et al. U.S. Pat. No. 4,916,429 and the solution proposed there comprises an optical sensor located so as to detect over-height and energize an alarm intended to warn the driver that his vehicle is over-height and he must stop before the canopy is struck and damaged. The problem with this solution is that all too often a vehicle over-height is due to a roof top load which is carried so seldom, that a driver, seeing or hearing the alarms, may not realize that they are directed at him, and he thus may continue to drive resulting in almost certain damage to the canopy and possibly also to the load and the vehicle.